As driving methods for driving a brushless motor, a driving method by intermittent turn-on and a driving method by continuous turn-on are known. In the driving method by continuous turn-on, the driving control is performed by a driving signal, which has a continuous waveform such as a sine wave, therefore, torque fluctuation is less than that of the driving method by intermittent turn-on, and as a result, there is a merit that it is possible to suppress occurrence of vibration and/or noises. In addition, when a driving voltage, which has a sine waveform, is used, it is possible to obtain high efficiency by performing a proper lead angle control to synchronize a phase of the induced voltage (counter electromotive force) with a phase of a phase current, because the waveforms of the induced voltage and the phase current become analogous.
In order to make the waveforms of the induced voltage and the phase current analogous in the driving method by continuous turn-on, it is necessary to correctly measure or estimate a position of the rotor in the motor and generate a driving signal according to the position of the rotor. It is possible to accurately detect the position of the rotor by using a rotary encoder, for example.
However, the rotary encoder is expensive, therefore, it is desirable that it is possible to accurately grasp the position of the rotor without using the rotary encoder. One of proposals for accurately grasping the position of the rotor without using the rotary encoder is a method for grasping the rough position of the rotor based on a sensor signal outputted from a hall effect Integrated Circuit to estimate a precise position between hall effect ICs based on the rotation speed of the rotor or the like (See Japanese Laid-open patent application Publication 2001-128483 (Patent Document 1).).
In addition, as described in Japanese Laid-open Patent Application Publication 2005-51950 (Patent Document 2), a method is also known, in which the waveform of the phase current is monitored in real time by using a current sensor, and a voltage command to a motor driving circuit (inverter) is generated based on the monitored waveform of the phase current. However, the current sensor is also expensive, therefore, it is desirable that the sine-wave driving control of the brushless motor is realized also without using the current sensor.